Add dependabot configuration

[matthiasbeyer]

@philippkeller it seems that also the "Require status checks to pass before merging" setting is not enabled on the repository. It needs to be enabled with "bors" as only required status check.

Can you please double-check this?

[philippkeller]

@matthiasbeyer I now added it, does it work now?

[matthiasbeyer]

Yes, seems like it!

bors single on bors merge

[bors[bot]]

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